There are two types of electromagnetic interference (EMI), i.e. transmission interference and radiation interference. The transmission interference is mainly the one mutually generated by the interference signal generated by an electronic device through a dielectric or common power supply line, and the radiation interference refers to the interference signal generated by an electronic device that is transmitted to another electric network or electronic device via spatial coupling. To prevent the electromagnetic interference generated by some electronic products from affecting or damaging the normal operation of other electronic devices, various governments or international organizations have proposed or established one after another some regulations or standards for the generation of electromagnetic interference, and the products conforming such regulations or standards are referred to as having electromagnetic compatibility (EMC).
Having or not electromagnetic compatibility is an important criterion in the examination of the qualification of an electronic product. Electromagnetic interference is an issue needing to be particularly paid attention to for some electronic devices using high-frequency signal in operation, i.e. those referred to as high-frequency signal generation devices in the present invention. For example, the electronic ballast used for driving such gas discharge lamps as fluorescent lamps, etc. in operation is a typical high-frequency signal generation device. In operation, electronic ballasts need usually to convert a conventional alternating current signal into a high-frequency alternating current signal via rectifying and inverter circuits to drive a fluorescent lamp, thus, the electromagnetic interference generated in such a process needs suppressing effectively.
The methods for suppressing electromagnetic interferences usually include filtering, shielding and earthing, etc. The method of filtering usually needs larger capacitors or inductors to be connected in the circuit, thus the cost is higher and the connection is complicated. In the methods of shielding, a reliable way of earthing is needed for desirable effects; when the earthing is not readily achieved, it is difficult to suppress the electromagnetic interference effectively using the methods of shielding and earthing and therefore, the application is limited.